1. Field of the Invention
This invention is related to a Kohler illumination. More specifically, the present invention provides a mirror assembly and method for producing an improved Kohler illumination.
2. Description of the Prior Art
Kohler illumination has frequently been used in a projecting optical system since it has conventionally given an image having a uniform illumination intensity. One example of such projecting optical system employing Kohler illumination system is a system for projecting images in an overhead projector.
An overhead projector includes a concave mirror, an illumination device as a light source, and an aspheric lens or a condenser. The aspheric lens or condenser is designed to have a much shorter focal length than is possible with a spherical glass lens of equal diameter and equal spherical aberration. It has one aspheric surface only, the other surface being either plano, spherical convex, or spherical concave. A correctly formed aspheric lens surface exactly cancels the spherical aberration that would otherwise be present in an optical system, or reduces both spherical aberration and coma to insignificance, in which case the system is conventional said to be aplanatic. Thus and as is well known, more energy can be concentrated into a small area, such as the entrance pupil of a projection lens system, or the sensitive area of a detector, than would otherwise be possible. Without the aspheric surface, marginal rays (which enter or exit the lens near its edge) may be severely aberrated and fail to contribute to the illumination of the desired target. The aspheric surface puts marginal rays on-target, and allows the potential collecting area of the system to be fully utilized. However, not all rays produced by the light source are reflected by the concave mirror for passage through the condenser. A substantial amount of light rays from the light source by-pass the concave mirror and are lost as is illustrated in the following U.S. patents which were discovered in the patentability investigation:
U.S. Pat. No. 3,848,122--titled: ADJUSTABLE LIGHT DIFFUSER FOR AN OPTICAL PROJECTION SYSTEM, Hutchison, III PA1 U.S. Pat. No. 4,163,150--titled: PROCESS AND APPARATUS FOR AUTOMATICALLY REALIZING KOHLER'S PRINCIPLE OF ILLUMINATION, Stankewitz PA1 U.S. PAT. No. 4,386,830--titled: INCIDENT LIGHT, BRIGHT-FIELD KOHLER ILLUMINATING DEVICE, Stankewitz et al PA1 U.S. Pat. No. 4,865,431--titled: ZOOM LENS SYSTEM FOR USE IN AN IMAGE PROJECTING APPARATUS WITH KOHLER ILLUMINATION, Ueda PA1 U.S. Pat. No. 4,909,615--titled: ZOOM LENS SYSTEM FOR USE IN AN IMAGE PROJECTING APPARATUS WITH KUM. / 0 /HLER ILLUMINATION, Ueda PA1 U.S. Pat. No. 5,548,373--titled: IMAGE READING APPARATUS, Ueda PA1 a) disposing a condenser against a housing supporting a mirror assembly with a light assembly supported by the housing and including a light member; PA1 b) illuminating the light member to produce light rays, a portion of which directly passes through an aspheric section of the condenser without being reflected, leaving a first residual light ray portion and a second residual light ray portion; PA1 c) reflecting the first residual light ray portion for passage through the aspheric section of the condenser; and PA1 d) reflecting the second residual light ray portion for passage through the condenser, preferably through a sloping condenser surface of the condenser.
U.S. Pat. No. 3,848,122 to Hutchison, III, teaches an adjustable light diffuser for an optical projection system. The diffuser is disclosed as comprising a support frame which supports a plurality of thin metallic wires which are partly suspended from frame. The plurality of metallic wires forms a spider web-like support means for adjustable screens which are similar to house hold screens.
U.S. Pat. No. 4,163,150 to Stankewitz discloses a process and apparatus for automatically realizing Kohler's principle of illumination in microscopes having a variable magnification system as well as an illumination system. FIG. 1 in this patent illustrates illumination in accordance of the Kohler principles wherein a source of light is reproduced by a collector condenser represented by a lens in an intermediate image plane and imaged by way of a mirror and optical system in a focal plane of a condenser assembly which consist of lenses and. In order to adjust the amount of light and the beam path within the entire optical system, a variable diaphragm is provided as best shown in FIG. 1. The diaphragm is arranged for adjusting the aperture and is placed in the focal plane of the condenser assembly or in the intermediate image of plane.
U.S. Pat. No. 4,386,830 to Stankewitz et al teaches an incident light, bright-field Kohler illuminating device for microscopes which reflect an illuminating beam into one half of the pupil of a microscope objective. The illuminating device comprises a diaphragm, a variable aperture diaphragm which is displaceable perpendicular to the optical axis, a condenser lens, a deflecting mirror, a polarizing filter, a field lens, and a beam deflecting optical element.
U.S. Pat. No. 4,909,615 to Ueda teaches a zoom lens system for use in an image projecting apparatus with the Kohler illumination. The beam lens system is taught as a comprising a first lens group and a second lens group. Both the first lens group and the second lens group are movable along an optical axis for zooming. An aperture stop is disposed between the first and second lens groups and. The aperture stop may be independently moved alone the optical axis so is to make the pupil position at the reduction side substantially invariable in an operation of zooming.
U.S. Pat. No. 4,865,431 to Ueda also relates to a zoom lens system for use in image projection apparatus with the Kohler illumination. This patent teaches an aperture stop which is movable relative to the movement of a first lens group and a second lens group in an operation zooming so is to make a pupil position of the zoom lens system at a reduction side invariable.
U.S. Pat. No. 5,548,373 to Ueda teaches an image reading apparatus for reading images recorded on light-transmitting film by means of a linear sensor array. The apparatus comprises an image reader having a plurality of reading elements arrayed in the main scanning direction. The apparatus also includes a light source and condenser elements having power characteristics such that the point of convergence recedes in accordance with the separation of the light rays from an optical access. The apparatus further comprises a projector for projecting a field image illuminated by the light source on the image reader. A drive member is provided for moving the pupil of the projector on an optical axis.
Therefore, what is needed and what has been invented is a system and method for capturing essentially all of the light rays produced by a light source for passage through a condenser or aspheric lens. What is more particularly needed and what has been invented is a mirror assembly including a condenser and method for capturing a substantial amount of light rays from a light source for passage through a condenser or aspheric lens for producing an improved Kohler illumination.